High-Spin States in Neutron-Rich Tantalum Isotopes

Description

Deep-inelastic reactions between ~6.2 MeV/A Xe-136 ions and a
thick W-186 target have been used to investigate excited states
in neutron-rich tantalum isotopes, establishing new structures in
Ta-182 and Ta-183. Spins and parities were determined from
angular correlations, internal conversion coefficients deduced
from intensity balances, and consideration of transition
strengths. The structure and configuration assignments have been
established from the...[Show more] measured magnetic properties and aligned
angular momenta of the observed rotational bands, as well as
comparison
with multi-quasiparticle calculations. Multi-quasiparticle
calculations using adjusted Nilsson energies and a Lipkin-Nogami
pairing model were performed for the series of tantalum nuclei
between A=181 and A=185.
The previously known 1300 ns isomer in Ta-183 was fully
characterised with its energy deduced as 1332 keV, a new lifetime
of 1200(140) ns, and a Kπ = 19/2+ assignment. Several new
multi-quasiparticle states, including two isomers with τ=41(5)
ns, Kπ = 29/2− and τ=70(4) ns, Kπ = 41/2−, have been
further identified and found to be feeding the 1200 ns isomer.
From the multi-quasiparticle calculations, a three-quasiparticle
configuration was assigned to both the Kπ = 19/2+ and 29/2−
isomers, while the upper-most isomer at 41/2− was established
to have a five-quasiparticle nature.
The out-of-beam γ ray coincidence data revealed a new level
structure in coincidence with tantalum X-rays. Measurements of
the magnetic properties of this structure from branching ratios,
together with the yields from various data sets using W-186,
Re-185, Re-187 and Os-192 targets, enabled an isotopic assignment
of this structure to Ta-182. Three isomeric states were found to
be feeding the known 10− isomer in Ta-182. A rotational band
built on the 10− isomer has been observed and is found to be
fed by a new 14+, 356 ns isomer at 1950 keV. Several intrinsic
states above this isomer are also identified. Firm spins and
parities have been established up to the 14+ isomeric state. Due
to limited spectroscopic information, the spins and parities for
the levels above this isomer are tentative.
The structure of Ta-182 and Ta-183 has also been investigated by
comparing the measured transition strengths with similar
transitions in the mass region. Most of the transition strengths
are in good agreement with expectations in the region,except for
a few cases of E1 decays that are anomalous. Possible
explanations for some of these anomalous transition strengths
have been discussed.
There are few cases where the predicted yrast states are not
observed experimentally in both Ta-183 and Ta-183 and possible
explanations are given.
Both the calculations and experimental results suggest a high
density of intrinsic states occurs along the yrast line at high
excitation energy in both nuclei. This pattern seems to emerge in
the more neutron-rich tantalum isotopes, resulting in a more
complex level scheme that is difficult to interpret.